Exhaust gas system

ABSTRACT

An exhaust system for the aftertreatment of exhaust gases of an internal combustion engine, having, in series as viewed in a flow direction of the exhaust gas, a catalyst for the oxidation of the exhaust gas and/or a catalyst for storing nitrogen oxides, having an introduction point for the feed of a reducing agent, having an SCR catalyst for the selective catalytic reduction of nitrogen oxides, and having a particle filter, wherein the particle filter is arranged downstream of the SCR catalyst in the flow direction, and a second SCR catalyst and/or an ammonia slippage catalyst is arranged downstream of the particle filter in the flow direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PCT Application PCT/EP218/058157, filed Mar. 29, 2018, which claims priority to German Patent Application 10 2017 206 425.0, filed Apr. 13, 2017. The disclosures of the above applications are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an exhaust system for the aftertreatment of exhaust gases of an internal combustion engine, having, in series as viewed in a flow direction of the exhaust gas, a catalyst for the oxidation of the exhaust gas and/or a catalyst for storing nitrogen oxides, having an introduction point for the feed of a reducing agent, having an SCR catalyst for the selective catalytic reduction of nitrogen oxides, and having a particle filter.

BACKGROUND OF THE INVENTION

In exhaust-gas aftertreatment systems, different types of catalysts are used in order to ensure the most effective and comprehensive possible conversion and filtering of the pollutants contained in the exhaust gas. Inter alia, use is made, for example, of catalysts which absorb and bind nitrogen oxides (NO_(x)). Furthermore, catalysts are also known which stimulate a conversion of nitrogen oxides to form less harmful reaction products. Furthermore, filters are known which filter particles of a certain size out of the exhaust gas.

A disadvantage of the devices known from the prior art for purifying exhaust gases of an internal combustion engine is that the arrangement of the individual catalysts in the exhaust tract is not optimal, and thus the individual catalysts do not achieve their individually optimum action. In particular, in devices known in the prior art, the so-called SCR catalysts for the selective catalytic reduction of nitrogen oxides are often arranged downstream of the particle filter. This has the result that the SCR catalyst cannot be operated at its optimum operating point.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide an exhaust system for the aftertreatment of exhaust gases of an internal combustion engine, which exhaust system has an optimized arrangement and furthermore an optimized configuration of the individual catalysts.

The object is achieved, with regard to the exhaust system, by means of an exhaust system having the features described below.

An exemplary embodiment of the invention relates to an exhaust system for the aftertreatment of exhaust gases of an internal combustion engine, having, in series as viewed in a flow direction of the exhaust gas, a catalyst for the oxidation of the exhaust gas and/or a catalyst for storing nitrogen oxides, having an introduction point for the feed of a reducing agent, having an SCR catalyst for the selective catalytic reduction of nitrogen oxides, and having a particle filter, wherein the particle filter is arranged downstream of the SCR catalyst in the flow direction, and a second SCR catalyst and/or an ammonia slippage catalyst is arranged downstream of the particle filter in the flow direction.

By virtue of an SCR catalyst being arranged upstream of the particle filter and an SCR catalyst being arranged downstream of the particle filter, a unit is created which functions more effectively. In particular owing to the large volume that particle filters generally have and owing to the cell geometries and cell densities that are required to permit adequate filtering of the exhaust gas, the operating conditions for SCR catalysts are impaired downstream of the particle filter.

This is for example owing to the fact that the temperature of the exhaust gas after flowing through the particle filter is greatly reduced, whereby the effect of the SCR catalyst is adversely affected. Furthermore, the particle filter may generate a large pressure loss, with the result that, at the downstream SCR catalyst, the exhaust gas no longer has a flow speed high enough to ensure optimum conversion. The integration of the SCR catalyst into a particle filter, for example by means of a coating of the particle filter in certain portions, is not optimal, because the commonly used cell densities and cell geometries of the honeycomb bodies of the particle filters are not optimal for the selective catalytic reduction of the exhaust gas in the SCR catalysts.

By means of an upstream SCR catalyst, an initial aftertreatment of the exhaust gas is performed, and in particular, with the aid of the reducing agent, typically formed by an aqueous urea solution, which is already introduced and evaporated, a conversion of the nitrogen oxides to form water and nitrogen is performed.

Purification of the exhaust-gas flow is realized by means of the downstream particle filter. An SCR catalyst positioned downstream of the particle filter may then convert nitrogen oxides that are still present in the exhaust gas. For this purpose, the second SCR catalyst may particularly advantageously have a cell geometry and/or cell density which differs from that of the first SCR catalyst, and may also have a different chemically active coating. It is thus for example possible to realize a conversion of the nitrogen oxides even in the presence of relatively low temperatures and flow speeds.

It is particularly advantageous if the first SCR catalyst, which is arranged upstream of the particle filter in the flow direction, is electrically heatable. The electric heating of the SCR catalyst is advantageous in order to heat the exhaust gas to the required operating temperature as quickly as possible in order to be able to perform the selective catalytic reduction.

It is also advantageous if the first SCR catalyst in the flow direction has an extent of at most 80 mm along the flow direction, wherein the SCR catalyst preferably has an extent of less than 50 mm, wherein the SCR catalyst particularly preferably has an extent of less than 40 mm. A short extent of the first SCR catalyst relative to the particle filter or the second SCR catalyst is advantageous in order that the inflow to the downstream particle filter is influenced only to a small degree, and a good filtering result in the particle filter is thus maintained.

A preferred exemplary embodiment is characterized in that the first SCR catalyst in the flow direction is arranged with a spacing to the particle filter along the flow direction of at most 20 mm, preferably of less than 10 mm and particularly preferably of less than 5 mm. The smallest possible spacing is advantageous in order to be able to combine the pressure loss generated owing to the first SCR catalyst and the particle filter. This facilitates the design of the exhaust system.

It is also preferable if the first SCR catalyst in the flow direction and the second SCR catalyst in the flow direction are formed as a modular unit with the particle filter. This is advantageous in order to facilitate the assembly process.

Furthermore, it is advantageous if the SCR catalysts and the particle filter are formed by a common honeycomb body. This is advantageous in order to minimize the number of installed elements, whereby the assembly process is simplified.

It is furthermore advantageous if the honeycomb body of the particle filter has a cell geometry and/or cell density which differs from that of the honeycomb body of the first SCR catalyst in the flow direction and from that of the honeycomb body of the second SCR catalyst in the flow direction. This is particularly advantageous in order to optimally adapt the individual components to their respective intended use and, overall, achieve the smallest possible pressure loss across the individual components. Furthermore, by means of the cell geometries, it is for example possible to influence the filter characteristics and the speed distribution.

It is also expedient if the cell geometry and/or the cell density and/or the chemically active coating of the honeycomb body and/or the amount of coating of the honeycomb body of the first SCR catalyst in the flow direction differs from the cell geometry and/or the cell density and/or the chemically active coating of the honeycomb body of the second SCR catalyst in the flow direction.

This is particularly advantageous in order to be able, for example in the region of the second SCR catalyst, to adapt the honeycomb body to the changed flow speed. A changed exhaust-gas composition is also to be expected in the region of the second SCR catalyst, because the latter is positioned downstream of the particle filter. It is thus possible, for example, to use a higher cell density and/or a reduced cell cross section without the risk of blockage of the flow channels formed in the honeycomb body. It is also possible for the coating to be advantageously adapted to the generally lower concentration of ammonia and nitrogen oxides, in order to nevertheless realize as comprehensive as possible a conversion of the nitrogen oxides.

The amount of coating of the first SCR catalyst in the flow direction is preferably greater than the amount of coating of the second SCR catalyst in the flow direction.

Advantageous developments of the present invention are described in the following description of the figures.

Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be discussed in detail below on the basis of an exemplary embodiment and with reference to the drawing. In the drawing:

FIG. 1 shows a sectional view through an exemplary exhaust tract with a multiplicity of catalysts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

FIG. 1 shows a sectional view through an exhaust tract of an exhaust system 1. The exhaust system 1 has a multiplicity of catalysts, which are arranged in casings and which are connected to one another by pipelines such that exhaust gas flows through the catalysts.

In the exemplary embodiment of FIG. 1, a catalyst 3 for the oxidation of the exhaust gas is arranged first in the flow direction 2 of the exhaust gas. Provided downstream of this is an introduction point 4 through which a reducing agent, such as for example an aqueous urea solution, is introduced into the exhaust tract. Arranged immediately adjacent to this introduction point 4 is an evaporation element 5, on which the reducing agent is evaporated in order to subsequently react, as ammonia, with the nitrogen oxides in the exhaust gas.

This is followed, downstream, by a first SCR catalyst 6, in which the nitrogen oxides in the exhaust gas react with the ammonia and the chemically active coating of the SCR catalyst 6 and thus form nitrogen and water.

Positioned downstream of the first SCR catalyst 6 is a particle filter 7, which serves for filtering the exhaust gas and which filters in particular soot particles out of the exhaust gas.

The particle filter 7 is followed by a second SCR catalyst 8. In terms of chemistry, the same reaction takes place in the second SCR catalyst as in the first SCR catalyst 6.

The advantage of such a construction lies in the fact that a reduction of the nitrogen oxides in the exhaust gas takes place already before the exhaust gas flows into the particle filter. The exhaust gas flowing into the first SCR catalyst is therefore still at a particularly high temperature, and also the exhaust-gas distribution over the cross section of the flow path has not yet been adversely affected by the particle filter. In this way, a particularly effective conversion of the nitrogen oxides in the exhaust gas may take place. Owing to the particle filter, which normally has a large volume, the temperature of the exhaust gas decreases considerably, as a result of which the chemical reaction in the downstream SCR catalyst normally does not take place optimally. The second SCR catalyst serves substantially for reducing that nitrogen oxide fraction which has not yet been reduced in the first SCR catalyst.

Instead of or in addition to the second SCR catalyst, an ammonia slippage catalyst may also be installed, which may bind excess ammonia (NH₃) that has not been converted during the reduction of the nitrogen oxides in the exhaust gas. In this way, it is for example possible to prevent a breakthrough of ammonia into the exhaust tailpipe, which could result in ammonia escaping into the surroundings and lead to an unpleasant smell or to contamination of the environment.

The exemplary embodiment in FIG. 1 is in particular not of a limiting nature, and serves for illustrating the concept of the invention.

The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention. 

What is claimed is:
 1. An exhaust system for the aftertreatment of exhaust gases of an internal combustion engine, comprising: a catalyst for the oxidation of the exhaust gas and/or a catalyst for storing nitrogen oxides, having an introduction point for the feed of a reducing agent; a first SCR catalyst for the selective catalytic reduction of nitrogen oxides, the SCR catalyst arranged downstream of the catalyst in a flow direction of the exhaust gas; a particle filter arranged downstream of the SCR catalyst in the flow direction; and a second catalyst arranged downstream of the particle filter in the flow direction.
 2. The exhaust system of claim 1, wherein the first SCR catalyst, which is arranged upstream of the particle filter in the flow direction, is electrically heatable.
 3. The exhaust system of claim 1, wherein the first SCR catalyst in the flow direction has an extent of at most 80 mm along the flow direction, the first SCR catalyst preferably has an extent of less than 50 mm, and the first SCR catalyst particularly preferably has an extent of less than 40 mm.
 4. The exhaust system of claim 1, wherein the first SCR catalyst in the flow direction is arranged with a spacing to the particle filter along the flow direction of at most 20 mm.
 5. The exhaust system of claim 1, wherein the first SCR catalyst in the flow direction is arranged with a spacing to the particle filter along the flow direction of less than 10 mm.
 6. The exhaust system of claim 1, wherein the first SCR catalyst in the flow direction is arranged with a spacing to the particle filter along the flow direction of less than 5 mm.
 7. The exhaust system of claim 1, wherein the first SCR catalyst in the flow direction and the second catalyst in the flow direction are formed as a modular unit with the particle filter.
 8. The exhaust system of claim 1, wherein the first SCR catalyst, the second catalyst, and the particle filter are formed by a common honeycomb body.
 9. The exhaust system of claim 8, wherein the honeycomb body of the particle filter has a cell geometry which differs from that of the honeycomb body of the first SCR catalyst in the flow direction and from that of the honeycomb body of the second catalyst in the flow direction.
 10. The exhaust system of claim 9, wherein at least one of the cell geometry or the chemically active coating of the honeycomb body or the amount of coating of the honeycomb body of the first SCR catalyst in the flow direction differs from at least one of the cell geometry or the cell density or the chemically active coating of the honeycomb body of the second catalyst in the flow direction.
 11. The exhaust system of claim 8, wherein the honeycomb body of the particle filter has a cell density which differs from that of the honeycomb body of the first SCR catalyst in the flow direction and from that of the honeycomb body of the second catalyst in the flow direction.
 12. The exhaust system of claim 11, wherein at least one of the cell density or the chemically active coating of the honeycomb body or the amount of coating of the honeycomb body of the first SCR catalyst in the flow direction differs from at least one of the cell geometry or the cell density or the chemically active coating of the honeycomb body of the second catalyst in the flow direction.
 13. The exhaust system of claim 8, wherein the honeycomb body of the particle filter has a cell geometry and cell density which differs from that of the honeycomb body of the first SCR catalyst in the flow direction and from that of the honeycomb body of the second catalyst in the flow direction.
 14. The exhaust system of claim 13, wherein at least one of the cell geometry or the cell density or the chemically active coating of the honeycomb body or the amount of coating of the honeycomb body of the first SCR catalyst in the flow direction differs from at least one of the cell geometry or the cell density or the chemically active coating of the honeycomb body of the second catalyst in the flow direction.
 15. The exhaust system of claim 1, the second catalyst further comprising a second SCR catalyst.
 16. The exhaust system of claim 15, further comprising an ammonia slippage catalyst arranged downstream of the second SCR catalyst.
 17. The exhaust system of claim 1, the second catalyst further comprising an ammonia slippage catalyst. 